1. Field of the Invention
The present invention relates to an uninterruptible power supply (UPS) system and more particularly to a UPS system having a simplified voltage detection circuit.
2. Description of the Related Art
UPS systems usually target at supplying power to downstream electric equipment when detecting an abnormal state of the mains power, such as power outage, surge or irregular high/low voltage, and are most frequently applied to telecommunication networks, private branch exchanges (PBXs), servers and the like requiring power to operate around the clock to avoid information and data loss, interruption in operation and even damage to equipment arising from the irregularity or failure of the mains power.
With reference to FIG. 9, a conventional UPS system is connected to a live line and a neutral line of the AC mains and has a system ground terminal. The conventional UPS system further has a main circuit 70, a frequency and voltage detection module 80 and a central controller 90.
The main circuit 70 is grounded through the system ground terminal and has a power input terminal set 71, a power output port 72, two switches 73, a mains power processing unit 74 and a backup power circuit 75. The power input terminal set 71 has a live terminal 711 and a neutral terminal 712 respectively connected to the live line and the neutral line of the AC mains. The live terminal 711 is connected to the backup power circuit 75 and the power output port 72 through one of the two switches 73 and the mains power processing unit 74. The neutral terminal 712 is connected to the backup power circuit 75 and the power output port 72 through the other switch 73.
The frequency and voltage detection module 80 is grounded through the system ground terminal, is connected to the live terminal 711 and the neutral terminal 712 of the power input terminal set 71 of the main circuit 70, detects the frequency and voltage of the mains power respectively through the live terminal 711 and the neutral terminal 712, and has a detected frequency output terminal 81 and a detected voltage output terminal 82.
The central controller 90 is connected to the two switches 73, the detected frequency output terminal 81 and the detected voltage output terminal 82 of the frequency and voltage detection module 80, has a ground terminal 91 grounded through the system ground terminal, and is built in with a frequency value range and a voltage value range. The central controller 90 detects a frequency value of the mains power and a voltage value between the live line and the neutral line of the AC mains through the frequency and voltage detection module 80. When determining that the frequency value is within the frequency value range and the voltage value is within the voltage value range, the central controller 90 controls the two switches 73 to switch on so that the AC mains supplies power through the power output port 72 and simultaneously charges the backup power circuit 75. When determining that the frequency value is not within the frequency value range or the voltage value is not within the voltage value range, the central controller 90 controls the two switches 73 to switch off and the backup power circuit 75 supplies power through the power output port 72.
With the foregoing UPS system, when the AC mains normally supplies power, the mains power is processed through the mains power processing unit 74 and then supplies power to an electric appliance connected to the power output port. The mains power processing unit is usually a voltage stabilizing circuit to lower or stabilize voltage of the mains power. When the AC mains is in an abnormal condition, such as power outage, surge, extraordinarily high/low voltage of high/low frequency, the backup power circuit 75 supplies power instead. Generally, the backup power circuit 75 has an AC and DC converter 751 and a rechargeable battery 752. When the mains power is normal, the mains power is converted into DC power by the AC and DC converter 751 to charge the rechargeable battery 752. When the backup power circuit 75 supplies power, the DC power supplied by the rechargeable battery 752 is converted into AC power by the AC and DC converter 751 to supply power. Hence, the power outputted from the power output port has stable frequency and voltage independent of the abnormal condition of the mains power. Although such UPS system achieves the effect of ensuring output power quality, the circuits of the frequency and voltage detection module 80 are so complicated that the components used are quite costly.
With reference to FIGS. 10A to 10D, the frequency and voltage detection module 80 is composed of a voltage detection unit 83 and a frequency detection unit 84. As the voltage of the neutral line of the AC mains is not equal to that of the system ground terminal, a voltage difference usually exists therebetween. To avoid the voltage difference causing deviation between a voltage detected by the central controller 90 and an actual voltage, input and output of the voltage detection unit 83 must be isolated, that is, the voltage level of the neutral line of the AC mains is taken by the voltage detection unit 83 as an input voltage level compared with the voltage level of the live line of the AC mains while the voltage level of the system ground terminal is taken as the output voltage level of the voltage detection unit 83. Consequently, the voltage detection unit 83 needs to be implemented by comparators having isolating effect and thus has a complicated circuit.
Moreover, to further ensure the quality of the voltage and frequency of the power outputted from the power output port 72, the UPS system further has an output power detection unit 76. The output power detection unit 76 usually has a voltage detection circuit and a current detection circuit. The voltage detection circuit and the current detection circuit are connected to the central controller 90, respectively detect voltage and current outputted from the power output port 72, and then output an output power calculated by the central controller 90. As a voltage difference exists between the system ground terminal of the UPS system and the neutral line of the AC mains, with reference to FIGS. 11A to 11D, the voltage detection unit of the output power detection unit 76 has two comparators 761 with isolating effect, and the output power detection unit 76 has a considerably complicated circuit and is costly.
With reference to FIG. 12, a conventional current detection circuit 762 usually serves to detect current of the neutral line of the AC mains. Similarly, as voltages of the neutral line of the AC mains and the system ground terminal deviate, the current detection circuit 762 should also provide isolating effect and has a current transformer 763 and a bridge rectifier 764. The current of the neutral line flows to two input terminals of the current transformer 763, is sensed by the current transformer 763, and outputted to the system ground terminal and the central controller 90 through the two output terminals of the current transformer 763 and the bridge rectifier 764. The current detection unit 762 also has a complicated circuit and the current transformer 763 is large in size and space-consuming.